A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
United States patent U.S. Pat. No. 6,542,220 B1 describes a lithographic apparatus, having at least one compartment closely surrounding at least one of the mask and substrate holders. During use, a purge gas is supplied to the compartment for flushing the compartment. The purge gas is transparent to the radiation of the projection beam. An air shower is provided above an area traversed by interferometer measurement beams, for interferometer conditioning.
United States patent U.S. Pat. No. 6,747,729 B2 describes a lithographic projection apparatus in which a purge gas is supplied to a space to displace ambient air from that space. Particularly, the mask and substrate stages are flushed with a specific gaseous composition having a refractive index identical to that of air under the same measuring conditions, so that interferometric displacement measuring devices are able to operate to a required degree of accuracy. For example, according to U.S. Pat. No. 6,747,729, the purge gas includes two or more components selected from N2, He, Ar, Kr, Ne and Xe.
European Patent EP 0 498 499 B1 discloses, among other things, the application of a constant, preferably laminar stream of air through the space in which interferometer beams propagate. To this aim, an air shower is applied. Both the purity and the temperature of the supplied air can be controlled. The stream of air provides an improved accuracy of a composite interferometer system of the lithographic apparatus.
Each of the above-mentioned patents U.S. Pat. No. 6,542,220 B1, U.S. Pat. No. 6,747,729 B2 and EP 0 498 499 B1 is incorporated in the present application in its entirety by reference.
Purge hoods and microenvironments may be used to prevent contamination of optics of the lithographic apparatus. For example, a certain optical performance is maintained using extremely clean dry air, for example, purified compressed air. Such air has a water vapor content of about 0%, or at least <1 ppb. This is advantageous, because water might serve as a catalyst for reactions on the optics when being irradiated with the projection beam, and such reactions deteriorate the functioning of the optics.
A problem with the use of purified dry air is that such dry air may hamper the operation of interferometer positioning devices of the apparatus. These positioning devices utilize interferometer beams which traverse spaces filled with common, usually water vapor containing, clean environmental air, see, for example, EP 0 498 499 B1. This wet clean air is usually circulated in large amounts from an environment of the apparatus, such as the commonly known ‘clean room,’ through respective areas of the apparatus and back to the clean room environment thereof. In that case, the circulated air has the same humidity as the air of the environment of the apparatus. Particularly, the dry purging air may leak from a respective microenvironment into the space through which interferometer beams traverse, locally leading to variations of the refractive index, which reduce the accuracy of the relevant interferometer measurements.